Boat hulls

ABSTRACT

Power boat comprises hull having a keel line running from bow to stern which defines an axis of symmetry from which port and starboard hull portions of bottom surface extend to meet side walls. The stern of the boat is defined by a transom. A trim plate is mounted for pivoting about a transverse axis to alter the relative position of the bow in the water and control the wake-making or wake-shaping capability. A flat inclined surface slopes upwardly and sternwards from the transition point to form a water flow modifying part of the hull bottom surface. By rotating trim plate about hinge axis so that its trailing edge is lowered, the water flow encountering plate is deflected down and so the stern of hull can experience lift and the bow will “squat” or lower somewhat, with the keel line assuming a progressively lesser angle of attack to the surface of the water. This is accompanied by a change in shape of the wake (and some increase in drag). The wake shaping can thus be controlled to allow for more entertaining wake boarding or to allow for different skill levels of wake boarders. Conversely, rotating trim plate to raise its trailing edge tends to raise the angle of attack.

FIELD OF THE INVENTION

The present invention relates to boat hulls, particularly of the planingtype.

BACKGROUND

Water sports including use of motor boats are popular activities. Atdifferent times depending on the activity it is desirable to adjust thetrim of a boat so that it travels through the water more smoothly or sothat it creates a desirable wake for a sporting activity astern of theboat such as water skiing (including slalom skiing) and wakeboarding.

Water skiing involves the use of one or two skis similar to snow skis bya person being towed on a line behind a planing boat. Wakeboarding issimilar except that a small board like a miniature surfboard is usedinstead of skis, and the aim is to make much more use of the disturbedwake generated by the boat in performing manoeuvres on the water and inthe air. Wakeboarders prefer the towing boat to generate comparativelylarge waves in the wake, by comparison to water skiers.

The present invention provides a new hull form that has been found to bevery suitable for adjusting the trim of the boat to suit or createdifferent water conditions. For example where choppy conditions areencountered, the trim of the boat can be adjusted to make the boattravel more smoothly through those conditions. Further the trim may beadjusted to create or modify the wake desired by wakeboardingenthusiasts or water skiers and provide for the degree and type ofwake-making to be readily controllable. For example in some sports it isdesirable to make the shape of the wake smooth rather than having a rolltop. The invention may also address a problem of boats that haveprojecting components at their sterns (as is the case with many boatsused for wakeboarding), namely difficulty encountered by persons in thewater in boarding the boat over its stern. A useful improvement inaccessibility and aesthetics may be provided in some embodiments of theinvention.

SUMMARY OF THE INVENTION

Accordingly the invention provides in one embodiment a boat having anadjustable trim, said boat including: (a) a hull with a bottom surface,side walls, transom, a keel line running from bow to stern defining anaxis of symmetry from which port and starboard hull portions extend tomeet said side walls, (b) a water flow modifying part of the hulllocated towards the stern comprising an inclined portion of the hullwhich extends up relative to said bottom surface in a direction towardsthe stern from a transition point along said keel line, and (c) locatedaft of said inclined portion a selectively moveable trim member with asurface for contacting water relatively flowing along said flowmodifying part and wherein said moveable trim member can be selectivelypositioned to increasingly obstruct said water flow and thereby lift thestern and lower the bow in the water when the boat is travellingforward.

When used herein, including in the claims, the expression “transitionpoint” is understood as including not only a discrete point where anabrupt change in the direction of the keel line occurs, but also asincluding a profiled, curved or stepped region forward and aft of whichthe direction of the keel line is different.

Preferably the inclined portion of the water flow modifying partprogressively widens from the transition point in the direction of thestern, however other shapes may be suitable. For example, although notpreferred, it is possible that the water flow modifying part of the hullfurther includes a flat surface which extends rearwardly in a directiontowards the stern from the inclined portion and which is substantiallyparallel to the port and starboard hull portions on either side thereof.

Preferably said inclined portion progressively widens to each andthereafter remains a predetermined width until reaching the trim member.Preferably also said inclined portion towards the trim member isrecessed below the port and starboard hull portions on either sidethereof.

Preferably the selectively moveable trim member comprises a trim plate,however any other suitable member having a body which can obstruct waterflow may be used.

In one possible embodiment the trim plate is moveable about a first axistransverse to the keel line. The axis may be substantially in the sameplane as the trim plate and for example, may be provided by a hinge bywhich the trim plate is mounted.

In another possible embodiment the axis is displaced above the keel lineand out of the plane of the trim plate. Preferably the trim plateextends from a drum or cylinder which is selectively rotatable about theaxis transverse to and above said keel line. The trim plate may bemounted so as to be substantially tangential to the drum or cylinderperiphery.

The degree to which the trim plate is extended into the water changesthe trim of the boat deflecting the water flow along the water flowmodifying part thus causing the stern to lift and the bow to lower intothe water. As further described in relation to illustrated embodiments,the trim member does not need to be of large dimensions. If a patch ofrough water is encountered then by lowering the trim plate perhaps by10% of its range of motion this can considerably improve the smoothnessof the passage of the boat through the water.

Also the size and shape of the wake created by the boat can bechangeable by moving the trim plate in the water flow along the waterflow modifying part of the hull.

Accordingly, in a second embodiment the invention provides a boat hullof adjustable trim comprising:

a hull body having a bottom surface with first and second portions andextending rearwardly from a bow to a stern of the hull body, the bottomsurface being substantially symmetrical on opposite sides of a verticalcentral plane that extends longitudinally of the hull body; and

a trim plate in the stern region rotatable by an actuator about an axisextending transversely of the hull body,

wherein—

-   (a) a keel line being a line of intersection between the said    central plane and the bottom surface has a first segment forward of    and a second segment rearward of a transition point partway along    the keel line, the second segment of said keel line extending    upwardly from said transition point and the second segment    terminating adjacent to a leading edge of a bottom surface of the    trim plate;-   (b) the first portion of the bottom surface of the hull extends    outwardly from the first segment of the keel line on each side of    the central plane; and-   (c) the second segment of the keel line being a centre line of the    second portion of the hull bottom surface, that second portion    extending outwardly from the keel line on each side of the central    plane.

It is possible to proportion the hull body and trim plate, and to adjustthe trim plate to control the shape of the wake suitable forwakeboarding.

Preferably, the said second portion of the hull bottom surface when seenin cross-section transverse to the central plane is substantially flator planar.

It is also preferred although not essential, that the said secondsegment of the keel line is substantially straight.

In one possible embodiment, the second portion of the hull bottomsurface extends transversely between steps that extend longitudinallyand are equally spaced apart on opposite sides of the central plane,port and starboard sections of the first portion of the hull bottomsurface being located transversely outboard of the said steps so thatthe first portion of the hull bottom surface extends from bow to stern,the said steps and the second portion over at least a part of its lengthdefining a recessed section or cutaway “tunnel” in the hull bottomsurface. The trim plate leading edge preferably then extends across thewidth of the second portion of the hull bottom surface and across thegap between the two steps.

In a particularly preferred embodiment, the leading edge of the trimplate bottom surface is positioned forwardly of the transom of the hullbody. The trailing edge of the trim plate may be forward or astern ofthe transom or may be longitudinally level with the transom. Having thetrim plate wholly or partially under the hull rather allows for easieraccess to the hull over its stern by persons in the water, because thetrim plate need not extend behind the transom of the hull body, or asfar behind the transom, as would otherwise be the case. It is preferredthat the trim plate does not extend rearwardly of the transom.

In a further aspect, the invention provides a boat having a hull bodyand trim plate as described herein, propulsion means, rudder means andactuators for adjusting the position of the trim plate.

BRIEF DESCRIPTION OF THE FIGURES

A more complete appreciation of the invention, and further inventivefeatures thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a side elevation of a planing boat according to the invention;

FIG. 2 is a perspective view from below of a stern portion of the hulland trim plate of the boat shown in FIG. 1;

FIG. 3 is a perspective view from below of a stern portion of a boathull with trim plate also according to the invention;

FIG. 4 is a perspective view from below of a stern portion of a boathull with trim plate according to a further embodiment of the invention;

FIG. 5 is a perspective view from below of a stern portion of the boathull as shown in FIG. 4 without the trim plate;

FIG. 6 is a longitudinal cross-section in the central plane of the sternportion of the hull and trim plate as shown in FIG. 4;

FIG. 7 is a longitudinal cross-section in the same plane as and from thesame viewpoint as FIG. 6 of a portion of the hull as shown in thatFigure, showing an alternative arrangement of a part of that hull;

FIG. 8 is a longitudinal cross-section in the same plane as and from thesame viewpoint as FIG. 6 of a portion of the hull as shown in thatFigure, showing a further alternative arrangement of a part of thathull.

FIG. 9 is a perspective view of an alternative possible configuration ofa trim plate mounted by a drum or cylinder.

FIG. 10 is a longitudinal cross section showing the trim plate and drumor cylinder of FIG. 9 in a boat hull of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a side elevation of a planing boat 1, such as would be usedfor wakeboarding, water skiing and the like.

Referring to the drawings generally boat 1 comprises hull 2 having akeel line 14 running from bow to stern which defines an axis of symmetryfrom which port and starboard hull portions 13 a and 13 b of bottomsurface 13 extend to meet side walls 12 a, 12 b. The stern of the boatis defined by transom 8.

Boat 1 has a hull 2, a propeller 3 driven by a propeller shaft 4supported by a skeg 5, and a rudder 6. A trim member shown as a plate 7is mounted to at the transom 8 of hull 2 for pivoting about atransversely extending hinge axis 9 in the manner shown by arrow “C”, byoperation of a linear actuator 10. Actuator 10 may be a hydrauliccylinder, for example, or an electric actuator of motor-and-screw type.

FIG. 2 shows the rear part of hull 2 seen from below, to illustrate howit is shaped to be able (with suitable adjustment of trim plate 7) toalter the relative position of the bow in the water and control thewake-making or wake-shaping capability. In FIG. 2, the skeg 5, propeller3, shaft 4, and rudder 6 are omitted for clarity. However, the positionof skeg 5 is shown by a phantom line 11, and trim plate 7 is shown. Hull2 is of hard chine type, with chines 12 c at the boundaries of sidewalls 12 a, 12 b with outer edges of bottom surfaces 13 a, 13 b thatextend outwardly and upwardly from a keel line 14. Immediately aft ofthe position of skeg 5, a flat inclined or ramp surface 15 is definedwhich slopes upwardly and sternwards from front edges 16. This is awater flow modifying part in this first embodiment of the invention. Atan intersection line 18, surface 15 meets a further flat surface 17 thatin side elevation (FIG. 1) is typically substantially parallel to keelline 14, forward of the transition point 5, 6. The term “transitionpoint” is intended herein (including in the claims) to mean the point orregion in the hull where the bottom surface turns up to form an inclinedsegment to form the water flow modifying part. The “point” need not be asharp or abrupt change in the line of the keel but may be a profiled orcurving transition or stepped region along a relatively small length ofthe keel line. The shape of surfaces 15 and 17 is emphasized by phantomlines 25, which show the line of intersection of the bottom of hull 2and trim plate 7 with a plane that extends vertically and fore-and-aftin hull 2 on its centerline. In effect, hull 2 is a conventional V-typehull, but with a “cut away” flow modifying part of its bottom surfacedefined by surfaces 15 and 17.

Trim plate 7 is secured at the transom 8 closely adjacent to the rearedge 19 of surface 17. As shown by arrows C in FIG. 1, trim plate 7 canbe selectively rotated so that its lower surface 20 lies in the sameplane as surface 17 or can be rotated from that position.

A user of boat 1 may alter its trim by rotating trim plate 7 about hingeaxis 9. In particular, if trim plate 7 is rotated so that its trailingedge 21 is lowered, the water flow encountering plate 7 is deflecteddown and so the stern of hull 2 can experience lift and the bow will“squat” or lower somewhat, with the keel line 14 assuming aprogressively lesser angle of attack to the surface of the water (notshown). This is accompanied by a change in shape of the wake (and someincrease in drag). The wake shaping can thus be controlled to allow formore entertaining wake boarding or to allow for different skill levelsof wake boarders. Conversely, rotating trim plate 7 to raise itstrailing edge 21 tends to raise the angle of attack.

Referring now to FIG. 3, there is shown a view, comparable to FIG. 2, ofa rear portion of a boat hull 50 that is a second embodiment of thepresent invention. Although the invention is not limited in itsapplication to hard chine boats, hull 50 is shown as being of this type,with chines 51, bottom surfaces 52, a keel line 53 and a transom 54. Asuitable skeg position is shown by phantom line 85. A flat inclined orramp surface 55 (corresponding to ramp surface 43 of hull 31 in FIG. 4)extends upwardly and rearwardly from keel line 53 from a transitionpoint 56 thereon behind skeg position 85 to a trailing edge 57 attransom 54. Close to trailing edge 57 is the leading edge 58 of a trimplate 59 which can be rotated about leading edge 58, or about ahorizontal transverse axis close and parallel to leading edge 58, toadjust the trim of hull 50. Hull 50 has no surface equivalent to surface17 of hull 2. This continuous extension of the ramp surface 55 to theleading edge of the trim plate 59 is believed preferable to the two partsurface (15, 17) in FIG. 1 because the wake height is created by thewater flow being “bent” upwardly by the ramp surface, whereas thetrailing float surface 17 in FIG. 1 may partially nullify the upflow.The trim plate modulates the effect of the ramp. Phantom lines 60 areshown on surface 55 and bottom surface 61 of trim plate 59 and are onthe centreline of hull 50. These show the cutaway nature of hull 50.

Ramp surface 55 extends back from two leading edges 62 defined by itsintersection with bottom surfaces 52. The provision of ramp surface 55amounts to a “bevelling” of the rear part of hull 50. Leading edges 62could be carried back to transom 54 and to do so would be within thescope of the invention. However, it has been found preferable to limitand maintain at a predetermined extent the width of ramp surface 55 andtrim plate 59. This is done by “bevelling” hull 50 so that it becomesrecessed below the port and starboard hull portions. That is, the rampsurface 55 extends between two upright fore-and-aft steps or planes 66at equal distances from the central plane of hull 50, as shown. Ineffect, two wedges (of which one, 63, is shown by phantom lines 67) areleft in hull 50, rather than being “bevelled” off.

It has been found possible with this arrangement to provide for improvedtrim in certain boating conditions or improved wake control (relative toa boat otherwise similar to boat 1) by rotating trim plate 59 so thatits rear edge 68 is moved below the position shown in FIG. 3 so that thestern of the boat is lifted and the bow of boat 30 rides lower in thewater and/or the boat 30 is of enhanced usefulness for wake boarding andany other activity where exaggerated wake making or wake control isdesirable.

By raising the rear edge 68 of trim plate 59, it is possible,conversely, to trim hull 50 so that its bow is higher than when it istrimmed for wake boarding, and so that drag and wake making is reduced.

Referring now to FIGS. 4, 5 and 6, there is shown a rear portion of aboat hull 31 formed according to another embodiment of the presentinvention. The view in FIGS. 4 and 5 is from a similar viewpoint as theview of hulls 2 and 50 in FIGS. 2 and 3 respectively, to simplifycomparison with those hulls. Hull 31 has chines 32, a keel line 33, mainbottom surfaces 34 and a transom 47. A trim plate 39 is mounted to hull31 for rotation about a transversely extending hinge axis 41 parallel toand close to the leading edge 42 of trim plate 39 so as to be rotatableby an actuator 40 about hinge axis 41, in the manner shown by arrow “D”.Hull 31 may be provided with a propeller, drive shaft, drive shaftsupport skeg, and a rudder, all of which may be similar to, andsimilarly located to, the corresponding parts of boat 1. For claritythese items are not shown in FIGS. 4, 5 and 6, although a skegattachment position is shown by a phantom line 37.

Corresponding to the ramp surface 55 in hull 50, hull 31 is providedwith an inclined surface 43. Surface 43 ramps upwardly and backwardlyfrom transition point 73 on keel line 33 behind skeg position 37, itsleading edges 79 being defined by the intersection of surface 43 and thehull bottom surfaces 34. Ramp surface 43 extends backward so that itsrear edge 44 lies closely adjacent to leading edge 42 of trim plate 39.That is, there is no surface corresponding to surface 17 of hull 2.

FIG. 5 shows, in a view similar to the views of FIGS. 2 and 3, the rearpart of hull 31 with propeller 35, shaft 36, skeg 37 and rudder 38omitted but trim plate 39 shown. FIG. 6 shows the same view of hull 31but with trim plate 39 now omitted also.

Ramp surface 43 extends upwardly and backwardly from transition point 73on keel line 33, and over part of its length is recessed between uprightfore-and-aft steps or surfaces 70, similar to surfaces 66 of hull 50.Hull 31 differs from hull 50 in that the trailing edge 44 of surface 43is closely adjacent to leading edge 42 of the bottom surface 74 of itstrim plate 39 and lies forward of its transom 47. Having a portion oftrim plate 39 forward of transom 47 enables hull 31 to be comparativelyeasily boarded by a person in the water astern of hull 31 as trim plate39 extends astern of transom 47 lesser than would otherwise be the case.In fact, the trim plate 39 need not extend astern as far as is shown inFIGS. 4 and 6. Trailing edge 44 could actually be forward of transom 47or in the same plane as transom 47 if required. Such an arrangementmaximises the advantage of ease of boarding over the stern.

Trim plate 39 is shown in FIGS. 4 and 6 as having its trailing edge 44behind transom 47.

The trailing edge 44 of surface 43 and the leading edge 42 of the bottomsurface 74 of trim plate 39 are close enough together over theirrespective lengths that bottom surface 74 of trim plate 39 extendssurface 43 backward with substantially no step at the transition betweenthe two, although of course depending on the angle to which trim plate39 is adjusted, the surfaces 43 and 74 may not be co-planar. To enablethis, and as best seen in FIGS. 5 and 6, a shoulder 75 is provided atthe rear of ramp surface 43, and trim plate 39 is positioned behindshoulder 75. Shoulder 75, planes 70 and a surface 76 of hull 31 define arecess 77 in which trim plate 39 is accommodated. Recess 77 isproportioned to accommodate a sufficient degree of upward hingingmovement of trim plate 39. A further recess 78 is provided in transom 47for the actuator 40.

Leading edges 79 of ramp surface 43 and leading edges 62 of ramp surface55 are shown as straight in FIGS. 3 and 4 because the hulls 50 and 31are hard-chine types with bottom surfaces 52 and 34 being substantiallyflat in their rear parts. However, the invention may be applied to boatswith rounded bottom surfaces as well. Moreover longitudinal strakes andthe like, such as are commonly used on vee-type hulls, may be providedon hulls embodying the invention.

Leading edges 62 and 79 are shown as sharp in the Figures, but may inpractice be somewhat faired into the local hull contour as desired.

Ramp surfaces 43 and 55 have been described as flat, but some curvature,as would be seen in a side view or in a longitudinal cross-section suchas FIG. 6, is not intended to be strictly precluded. Ramp surfaces 43and 55 have been described as continuing back to the leading edges oftrim plates 39 and 59 respectively, with no intervening separate surfacecomparable to the surface 17 between surface 15 and trim plate 7 of hull1 in FIG. 1. However, an intervening surface similar to surface 17 thatin a longitudinal direction is small by comparison to the trim plate 39,59 and ramp surface 43, 55 it adjoins is not intended to be precluded.

FIG. 6 shows a flange 80 a (of a gland, not shown in full) where arudder shaft (not shown) passes through ramp surface 43, flange 80 abeing flush with surface 43. However, as it may be desirable to avoid alifting force for the rudder shaft axis 87 to be perpendicular tosurface 43. FIGS. 7 and 8 (in which arrows point to the bow of the hull31) show a flange 80 that is perpendicular to shaft 82 where shaft 82 ispassed through the ramp surface 43. It is preferred, so as to reducepurely parasitic drag (i.e. drag that does not contribute significantlyto wave making), that the lower front edge 81 of flange 80 not protrudefrom surface 43. The lower front edge 81 may therefore be madesubstantially flush with surface 43 as shown in FIG. 7. Generally,depending on the angle of rudder shaft 82 to surface 43, this will meanthat the rear lower edge 83 of flange 80 will not be flush with surface43. An alternative arrangement is shown in FIG. 8, where the rear loweredge 83 of flange 80 is made substantially flush with surface 43. Inthis case, flange 80 sits in a small recess 84 in surface 43. It isthought that the arrangement in FIG. 8 may be advantageous.

In the embodiment of FIGS. 9 and 10, the trim plate 91 is mounted by aselectively rotatable drum or cylinder 95 so that the transverse axis 96of the cylinder 95 about which the trim plate 91 moves is displacedabove the keel line 14 and out of the plane of the trim plate 91. Asshown the trim plate 91 is mounted so that it is substantiallytangential to the cylinder 95. As the cylinder 95 rotates about axis 96by an actuator mechanism (not shown) as shown by arrows E, the trimplate 91 moves from a retracted position shown in FIG. 10 in which it isan extension of the plane of the inclined portion 43 into progressivelylower positions 91 a, 91 b shown in broken line in which it projectsinto the water flow along the inclined portion 43. The dimension of thetrim plate 91 can be relatively modest, e.g. about 300 mm in thetransverse dimension and about 10 mm in width (i.e. the extent to whichit projects down into the water flow). Although small in dimensions,such a trim plate 91 can be very effective in modifying the trim of theboat and controlling the wake's properties. A suitable manual actuatormechanism can include a cable extending from the boat driver's stationwhere a manually movable handle is provided for the driver to operatethe trim mechanism. The cable can operate a linkage coupled to thecylinder 95 for rotating it about its axis 96. With such a simplemechanism, the position of the trim plate 91 can be known by the driverfrom the position of the movable handle. Only small load is experiencedby the mechanism as a result of the water flow encountering the smallplate 91 and being deflected thereby, so powered assistance fromhydraulics or electric drive is unnecessary.

In use of a boat utilising the present invention the position of thetrim member can be adjusted by the boat driver as desired for control atspeed and/or for wake control. For example, at high speed (say 60 km perhour or more), a small movement only of the trim member into the waterflowing along the flow modifying part of the hull can lift the stern anddrop the bow which can be needed upon meeting rough or choppy water.When using the boat towing a wake boarder (typically about 40 km perhour) the trim plate can be positioned at about 45° for optimum shape ofthe wake.

It is to be understood that various alterations, modifications and/oradditions may be made to the features of the possible and preferredembodiment(s) of the invention as herein described without departingfrom the spirit and scope of the invention.

1. A boat having an adjustable trim, said boat including: (a) a hullwith a bottom surface, side walls, transom, a keel line running from bowto stern defining an axis of symmetry from which port and starboard hullportions extend to meet said side walls, (b) a water flow modifying partof the hull located towards the stern comprising an inclined portion ofthe hull which extends up relative to said bottom surface in a directiontowards the stern from a transition point along said keel line, and (c)located aft of said inclined portion a selectively moveable trim memberwith a surface for contacting water relatively flowing along said flowmodifying part and wherein said moveable trim member can be selectivelypositioned to increasingly obstruct said water flow and thereby lift thestern and lower the bow in the water when the boat is travellingforward.
 2. The boat of claim 1 wherein said inclined portionprogressively widens from said transition point in the direction of thestern.
 3. The boat of claim 2 wherein said inclined portionprogressively widens to each and thereafter remains a predeterminedwidth until reaching the trim member.
 4. The boat of claim 1 whereinsaid inclined portion towards the trim member is recessed below the portand starboard hull portions on either side thereof.
 5. The boat of claim1 wherein the water flow modifying part of the hull further includes aflat surface which extends rearwardly in a direction towards the sternfrom the inclined portion and which is substantially parallel to theport and starboard hull portions on either side thereof.
 6. The boat ofclaim 1 wherein the selectively moveable trim member comprises a plate.7. The boat of claim 6 wherein said plate is moveable about an axistransverse to said keel line.
 8. The boat of claim 7 wherein the axis issubstantially in the same plane of the trim plate.
 9. The boat of claim8 wherein said transverse axis in provided by a hinge by which the trimplate is mounted.
 10. The boat of claim 7 wherein the axis is displacedabove the keel line and out of the plane of the trim plate.
 11. The boatof claim 10 wherein said plate extends from a drum or cylinder which isselectively rotatable about the axis transverse to and above said keelline.
 12. The boat of claim 11 wherein said plate is mounted so as to besubstantially tangential to the drum or cylinder periphery.
 13. A boathull of adjustable trim comprising: a hull body having a bottom surfacewith first and second portions and extending rearwardly from a bow to astern of the hull body, the bottom surface being substantiallysymmetrical on opposite sides of a vertical central plane that extendslongitudinally of the hull body; and a trim plate in the stern regionrotatable by an actuator about an axis extending transversely of thehull body, wherein— (a) a keel line being a line of intersection betweenthe said central plane and the bottom surface has a first segmentforward of and a second segment rearward of a transition point partwayalong the keel line, the second segment of said keel line extendingupwardly from said transition point and the second segment terminatingadjacent to a leading edge of a bottom surface of the trim plate; (b)the first portion of the bottom surface of the hull extends outwardlyfrom the first segment of the keel line on each side of the centralplane; and (c) the second segment of the keel line being a centreline ofthe second portion of the hull bottom surface, that second portionextending outwardly from the keel line on each side of the centralplane.
 14. The boat of claim 13 wherein said second portion of thebottom surface is substantially flat or planar.
 15. The boat of claim 13wherein the second portion of the hull bottom surface extendstransversely between steps that extend longitudinally and are equallyspaced apart on opposite sides of the central plane, sections of thefirst portion of the hull bottom surface being located to port andstarboard of the said steps so that the first portion of the hull bottomsurface extends from bow to stern, the said steps and the second portionover at least a part of its length defining a recessed surface in thehull bottom surface between the port and starboard sections.
 16. Theboat of claim 15 wherein the trim plate leading edge extends across thewidth of the second portion of the hull bottom surface and across thegap between the two steps.
 17. The boat of claim 13 wherein theselectively moveable trim a plate is moveable about an axis transverseto said keel line.
 18. The boat of claim 17 wherein the axis issubstantially in the same plane of the trim plate.
 19. The boat of claim13 wherein said trim plate extends from a drum or cylinder which isselectively rotatable about the axis transverse to and above said keelline.
 20. The boat of claim 19 wherein said plate is mounted so as to besubstantially tangential to the drum or cylinder periphery.